Considerable effort has been devoted for developing rapidly and widely tunable wavelength laser sources for optical reflectometry, biomedical imaging, sensor interrogation, and tests and measurements. A narrow line width, wide-range and rapid tuning have been obtained by the use of an intra-cavity narrow band wavelength scanning filter. Mode-hopping-free, single-frequency operation has been provided in an extended-cavity semiconductor laser by using a diffraction grating filter arrangement. To obtain a single-frequency laser operation and to ensure a mode-hop-free tuning, however, a complicated mechanical apparatus and limit the maximum tuning speed may need to be used, conventionally. One of the fastest tuning speeds demonstrated using the conventional systems has been limited to less than 100 nm/s. In certain applications, such as biomedical imaging, multiple-longitudinal mode operation, corresponding to an instantaneous line width as large or great than 10 GHz, may be sufficient. Such width can provide a ranging depth of a few millimeters in tissues in optical coherence tomography and a micrometer-level transverse resolution in spectrally-encoded confocal microscopy.
A line width on the order of about 10 GHz is achievable with the use of an intra-cavity tuning element (such as, e.g., an acousto-optic filter, Fabry-Perot filter, and galvanometer-driven diffraction grating filter). However, the sweep frequency previously provided has been less than about 1 kHz, limited by finite tuning speeds of the filters. Higher-speed tuning with a repetition rate greater than 15 kHz may be needed for video-rate (e.g., greater than 30 frames/second), high-resolution optical imaging in biomedical applications.
Further, a wavelength-swept laser has been described which can use polygon scanning filter, and that can provide high-speed wavelength tuning up to about 10,000 nm/ms. While the high-speed polygon based wavelength-swept light source facilitates high-speed imaging as fast as about 200 frames/s, wavelength tuning rate as fast as about 10,000 nm/ms, maintaining an instantaneous line-width narrower than 0.15 nm has already reached to the limit of the current polygon based wavelength-swept filter.
Indeed, one of the objects of the exemplary embodiments of the present disclosure is to reduce or address the deficiencies and/or limitations of the prior art procedures and systems described herein above. For example, with respect to faster tuning, wide wavelength tuning range and narrow instantaneous line-width at fast tuning rate, there may be a need for an exemplary embodiment of wavelength scanning filter arrangement and procedure (e.g., a laser procedure).